The invention relates to a method for determining in a positioning system the time of reception of a beacon signal received by a receiver receiving and tracking signals from at least one beacon, wherein signals from this beacon have a component of a known regularity. The invention relates equally to a corresponding receiver and to a positioning system comprising a receiver.
A well known positioning system which is based on the evaluation of signals transmitted by beacons is GPS (Global Positioning System). The constellation in GPS consists of more than 20 satellites employed as beacons that orbit the earth.
Each of the satellites, which are also called space vehicles (SV), transmits two microwave carrier signals. One of these carrier signals L1 is employed for carrying a navigation message and code signals of a standard positioning service (SPS). The L1 carrier phase is modulated by each satellite with a different C/A (Coarse Acquisition) Code. Thus, different channels are obtained for the transmission by the different satellites. The C/A code, which is spreading the spectrum over a 1 MHz bandwidth, is repeated every 1023 bits, the epoch of the code being 1 ms. The carrier frequency of the L1 signal is further modulated with navigation information at a bit rate of 50 bit/s, which information comprises in particular ephemeris and almanac data. Ephemeris parameters describe short sections of the orbit of the respective satellite. Based on these ephemeris parameters, an algorithm can estimate the position of the satellite for any time while the satellite is in the respective described section. The orbits calculated using ephemeris parameters are quite accurate, but the ephemeris parameters are only valid for a short time, i.e. for about 2-4 hours. The almanac data contain in addition coarse orbit parameters. The orbits calculated based on almanac data are not as accurate as the orbits calculated based on ephemeris data, but their validity time is longer than one week. Almanac and ephemeris data also comprise clock correction parameters which indicate the current deviation of the satellite clock versus a general GPS time.
Further, a time-of-week TOW count is reported every six seconds as another part of the navigation message.
A GPS receiver of which the position is to be determined receives the signals transmitted by the currently available satellites, and a tracking unit of the receiver detects and tracks the channels used by different satellites based on the different comprised C/A codes. The receiver first determines the time of transmission TOT of the code transmitted by each satellite. Usually, the estimated time of transmission is composed of two components. A first component is the TOW count extracted from the decoded navigation message in the signals from the satellite, which has a precision of six seconds. A second component is based on counting the epochs and chips from the time at which the bits indicating the TOW are received in the tracking unit of the receiver. The epoch and chip count provides the receiver with the milliseconds and sub-milliseconds of the time of transmission of specific received bits.
Based on the time of transmission and the measured time of arrival TOA of the signal at the receiver, the time of flight TOF required by the signal to propagate from the satellite to the receiver is determined. By multiplying this TOF with the speed of light, it is converted to the distance between the receiver and the respective satellite. The computed distance between a specific satellite and a receiver is called pseudo-range, because the GPS system time is not accurately known in the receiver. Usually, the receiver calculates the accurate time of arrival of a signal based on some initial estimate, and the more accurate the initial time estimate is, the more efficient are position and accurate time calculations. A reference GPS time can, but does not have to be provided to the receiver by a network.
The computed distances and the estimated positions of the satellites then permit a calculation of the current position of the receiver, since the receiver is located at an intersection of the pseudo-ranges from a set of satellites. In order to be able to compute a position of a receiver in three dimensions and the time offset in the receiver clock, the signals from four different GPS satellite signals are required.
If navigation data are available on one of the receiver channels, the indication of the time of transmission comprised in a received signal can also be used in a time initialization for correcting a clock error in the receiver. In GPS, an initial time estimate is needed for the positioning. For the initial time estimate, the average propagation time of satellite signal of around 0.078 seconds is added to the time of transmission extracted from the navigation information. The result is used as initial estimate of the time of arrival of a signal, which estimate lies within around 20 ms of the accurate time of arrival. The receiver then determines for different satellites the time at which a respective signal left the satellite. Using the initial estimate of the current time, the receiver forms pseudorange measurements as the time interval during which the respective signal was propagating from the satellite to the receiver either in seconds or in meters by scaling with the speed of light. After the position of the receiver has been calculated from the determined pseudoranges, the accurate time of reception can then be calculated from standard GPS equations with an accuracy of 1 xcexcs.
However, in order to be able to make use of this time initialization, the navigation data from a satellite signal is needed. Currently, most of the GPS receivers are designed for outdoor operations with good signal levels from satellites. Thus, only good propagation conditions ensure that the navigation data required for the described time initialization is available.
In bad propagation conditions, in contrast, it may not be possible to extract the navigation message accurately enough from received satellite signals, since a high bit-error rate and weak signal levels make a robust decoding of navigation bits impossible. Such bad propagation conditions, which are often given indoors, render the time initialization and the pseudorange measurements more difficult.
For those cases, in which the standard time initialization methods can not be applied since the navigation data are noisy, the time initialization process for the receiver can be performed by a time recovery method. Some known time recovery methods are based on the cross-correlation of the tracked signal and an expected signal to define the time of transmission, as will be explained in the following.
Even in bad propagation conditions, the receiver is often still able to track the signal of a GPS satellite and to provide raw data without an evaluation of the contained bit values. On the other hand, the navigation message containing ephemeris and/or almanac data may be extractable from satellite signals received by a base station of a mobile communications network.
The raw satellite signal is received at the receiver from the sky and the receiving time of the data is read from a time counter of the receiver. If the receiver is part of a mobile station or connected to a mobile station, the raw data can then be sent from the mobile receiver to a mobile communication network. In the network, the raw data is cross-correlated with corresponding data received by an Location Measurement Unit (LMU), which is a GPS receiver located in the network. The time of transmission of the tracked signal can then be estimated based on the navigation data in the signal received by the network which results in the highest correlation. An initial estimate for the time of reception is the LMU time of reception of the same fragment. The LMU sends the determined time estimate to the mobile receiver. After the position of the receiver has been calculated from the determined pseudoranges to at least four GPS satellites, the accurate time of reception can now be calculated from standard GPS equations with an accuracy of 1 xcexcs.
It is a disadvantage of this approach that the computing for the time initialization has to be carried out in a network. It also requires additional signaling between the mobile receiver and the network.
It is an object of the invention to provide a possibility of determining a quite accurate time of reception of a beacon signal received by a receiver receiving and tracking signals from at least one beacon. It is in particular an object of the invention to provide such a possibility for the case that a comprised indication of the time of transmission of the signal cannot be extracted from the signal. It is further an object of the invention to enable a determination of a quite accurate time of reception that can be based on the signals from a single beacon. It is moreover an object of the invention to enable a determination of a quite accurate time of reception that can also be realized independently of a mobile communication network, e.g. in the receiver itself.
These objects are reached according to the invention with a method for determining in a positioning system the time of reception of a signal transmitted by a beacon and received by a receiver receiving and tracking signals at least from this beacon. It is assumed for the proposed method that the signals from the beacon have a component of a known regularity, like a GPS signal. In a first step of the proposed method, a beacon signal is reconstructed for an interval based on available information on signals from the beacon, e.g. ephemeris and/or almanac data, which are valid for this interval. In a second step, a cross-correlation is performed between the respectively overlapping parts of the reconstructed beacon signal and the received beacon signal at different relative positions to each other. It is to be noted that in case the received signal and the reconstructed signal have a different sampling rate, this difference has to be taken into account for the cross-correlation.
In a third step, the time of transmission of the received beacon signal is determined based on information for the reconstructed signal and on the relative position resulting in the maximum correlation value between the reconstructed signal and the received signal. In a last step of the proposed method, a time of reception of the received beacon signal is determined as the sum of said determined time of transmission of the received beacon signal and of a calculated time of flight of the received beacon signal.
It is to be noted that in addition the correlation values closest to the maximum correlation value might be considered for determining the time of transmission of the received beacon signal, in particular in case there is no clear maximum value.
The objects of the invention are also reached with a receiver comprising means for receiving and tracking signals from at least one beacon and processing means for realizing the proposed method.
The objects of the invention are further reached with a positioning system comprising a receiver and at least one network element of a network. This network may be a mobile communication network or any other network. The receiver comprises again means for receiving and tracking signals from at least one beacon and processing means for realizing the steps of the proposed method. In addition, the receiver comprises means for communicating with the network.
Finally, the objects of the invention are reached according to the invention with a positioning system, in which the steps of the proposed method are realized by a processing unit of the system which is external to a receiver of the system. The receiver includes in this case means for receiving and tracking signals from at least one beacon and means for providing received and tracked beacon signals to the processing unit. The processing unit can also include other functions. It can be given e.g. by a mobile station to which the receiver is connected and which is able to communicate with a mobile communication network for receiving pieces of information. It can also be given by a network element of a network, in which required pieces of information are available.
The invention proceeds from the idea that it is possible to reconstruct a beacon signal expected in a specific interval, proceeding from information available for this interval for signals from the beacon in question. A GPS signal comprising navigation data can be built for example for a desired period of time using available ephemeris and/or almanac data. A reconstructed beacon signal can then be cross-correlated with the raw data of a received signal, which raw data might be so noisy that the comprised information can not be extracted correctly. The cross-correlation reveals the best match between the received signal and the reconstructed signal in different shifting positions. Due to the regularity of the beacon signals, the interval does not have to coincide with the time of the received signal in question in order to enable a good match, but only with a corresponding signal with respect to the regularity. The time of transmission of the received signal can be determined based on information available for the part of the reconstructed signal resulting in the best match, e.g. the bit address of the last bit in this part of the reconstructed signal. The desired time of reception of the received signal is then given by the sum of the time of transmission and the time of flight required by the signal to propagate from the beacon to the receiver.
It is thus an advantage of the invention that it enables an estimation of the time of reception of a beacon signal with a high accuracy in a situation in which information comprised in a received beacon signal is not extractable. It is moreover an advantage of the invention that the time of reception can be determined based on the estimated correct time of transmission of a single beacon, while the GPS equations employed by conventional methods require to this end the time of transmission of signals of at least four GPS satellites. It is equally an advantage of the invention that the time of reception can be determined outside of a network, if desired. Still, a microcontroller unit (MCU) of a network can assist the baseband of the receiver for a fast re-acquisition of the system time.
It is to be noted that in case ephemeris, almanac and other components of a navigation message are available at the processing means, e.g. from a network, it is usually possible to reconstruct the signal completely. However, if only ephemeris is available, or only almanac, then a reconstruction can be done partially by replacing xe2x80x9cunknownxe2x80x9d bits with 0s and reconstructed bits by xc2x11s. A control can be maintained in the receiver during each cross-correlation by monitoring the number of xe2x80x9cnot reconstructedxe2x80x9d bits having a value of xe2x80x9c0xe2x80x9d. If that number is not big, the cross-correlation is performed, but if the reconstructed array is almost empty, this fragment is not used and the receiver will wait for a more favorable moment. Since different phases are compared by sliding and cross-correlating, the cross-correlation peak value depends on the number xe2x80x9cunknownxe2x80x9d bits at the given stage, which number changes from one sliding position to the next. A kind of scaling may be used to normalize properly, so that the method according to the invention still works normally even with some unkown bits.
The interval for which a signal is reconstructed may be selected in particular such that it can be expected to comprise the last bit edge received before reception of the received beacon signal of which the time of transmission is to be determined.
The accurate time when a received signal was transmitted by a beacon can be computed based on an identification associated to the bits of the reconstructed signal, which enables a determination of the time at which they would have been transmitted by the beacon. Then, only the time difference between the assumed reception of a specific bit of the reconstructed signal and the reception of said received beacon signal is required as further information, in order to determine the accurate time of transmission.
The time of flight of the beacon signal, which is required in addition for determining a relatively accurate time of reception of the beacon signal, can be estimated based on an available position of the beacon at the accurate time of transmission of the received signal and on an inaccurate reference position of the receiver. Such an inaccurate reference position can be provided for instance by a network, and could be e.g. 30 km away from the correct position. The reference position could also be some earlier calculated position of the receiver or any other estimate of the position, which lies up unto 30 km away from the correct position. The resulting time of flight will then be within 1 ms from the correct value. The time of reception determined based on this time of flight lies equally within 1 ms from the correct value.
Advantageously, the received signal is bit-synchronized before the correlation according to the invention is performed, for identifying bit edges. This also enables a correct alignment of samples in each relative position between the received signal and the reconstructed signal.
The cross-correlation can be carried out in a conventional way, or with an approach compensating for residual sinusoidal modulations in the received and tracked beacon signal.
In case the receiver is able to communicate with a network, the receiver may receive various information as basis for the calculations according to the invention. It is to be noted that the receiver can be able to communicate with the network either directly or indirectly, in the case of a mobile communication network for instance via some mobile station. A network may provide a receiver for example with a reference time for the receiver, with a maximum error of this reference time, with a reference position of the receiver and with position information for at least one beacon. The position information can include in particular ephemeris data and/or almanac data for at least one beacon. In an advantageous embodiment of the positioning system according to the invention including a network element of a mobile communication network, the network element comprises therefore means for receiving and tracking signals from at least one beacon, and moreover means for providing the receiver with at least one of the above mentioned pieces of information. As mentioned above, the network providing assistance data can be a mobile communication network, but it can also be any other kind of network which is capable of providing assistance data via a network element, e.g. via a DGPS (Differential Global Positioning system) station.
Each of these data may alternatively be stored in the receiver or be provided by some algorithm in the receiver or a connected processing unit, e.g. another time-recovery algorithm providing an estimate of the current time and the maximum possible error in this estimate. Thus, a receiver according to the invention can also operate independently of assistance data from a network.
Preferably, though not necessarily, the method according to the invention is implemented as software.
The invention can be employed for a fast re-acquisition and for determining the time estimate. By this system, signals of 20 dBHz or below can be acquired. During fast re-acquisition, the time estimate is used to predict the code-phases and Doppler frequencies of other beacons and to narrow the search to fewer candidates.
The beacon can be in particular, though not exclusively, a satellite or a base station of a mobile communication network.
The invention can be employed in particular in the current GPS system, but equally in future extended GPS systems with new signals and in other similar beacon based positioning systems such as Galileo.
Preferably, though not necessarily, the receiver is a GPS receiver and the beacon is a GPS space vehicle.